2.4 , error with virtual plugin

Hello,

even with updated config for virtual plugin:

mail_plugins { virtual = yes }

namespace virtual { prefix = virtual. separator = . mail_driver = virtual mail_path = ~/virtual }

I get this error:

imap(lonestar@lonestar.it)<2991><64ab1o8sDJl/AAAB>: Error: Namespace virtual: Unknown mail storage driver virtual imap(lonestar@lonestar.it)<2991><64ab1o8sDJl/AAAB>: Info: Disconnected: Namespace virtual: Unknown mail storage driver virtual in=0 out=442 deleted=0 expunged=0 trashed=0 hdr_count=0 hdr_bytes=0 body_count=0 body_bytes=0

What could it be?

-- Ciao, Luigi